1. Field of the Invention
The current invention relates to a security element for security substrates, such as those used for banknotes and the like, having enhanced public recognition, anti-counterfeit and detection properties.
2. The Prior Art
It is widely known to include security elements, such as security threads or strips, into security documents to protect against forgery. Typically these elements comprise a polymer carrier to which a metal layer is applied, though they may also have additional functional layers such as magnetic, thermochromic or luminescent layers.
Security elements of this type have the advantage that they can be verified both visually and by machine. However, there is a constant need to improve the security of these devices in order to remain one step ahead of the counterfeiters. Developments have included improving the public recognition, machine readable, and anti-counterfeit properties.
Public recognition or visually identifiable features have been known for some time and include security elements with microprint; metallic indicia on a transparent element (such as are disclosed in EP-A-279880, U.S. Pat. No. 4,941,017 & U.S. Pat. No. 4,652,015) and metallic security elements with transparent indicia (such as are disclosed in EP-A-0319157). The latter security elements, supplied by the De La Rue Group, are known by the trade mark Cleartext®, and are utilised in a number of the world's major currencies. Cleartext® security elements have proved to be highly successful due the ease with which the public can verify them without the need for additional aids or equipment. A variant of the Cleartext® type of security element is described in EP-A-659587, in which the security elements are provided with demetallised characters of varying heights.
The principles behind Cleartext® and this type of security element have been further improved by enhancing both the anti-counterfeit and aesthetic properties as described in EP-A-972111. In this specification the security element design comprises at least one repeating geometric pattern of which one of more of the frequency, instantaneous amplitude and/or maximum amplitude of the pattern varies along the length of the element and design having at least one non-linear boundary. Such designs are much harder to counterfeit and consequently more secure. They also have the additional benefit of being highly aesthetic and can be designed in such a way as to co-ordinate with other design features on a document.
It has also been recognised that there is an increasing need to be able to identify and characterise security documents automatically by machine. This can be achieved by the provision of additional functional layers to security elements as described earlier. It is particularly common to make use of magnetic layers and, more recently, coded magnetic layers. One such coded security element is described in EP-A-0407550. Here the code is provided in the form of a machine-readable binary code in the magnetic layer. The code consists of alternating ‘termination’ and ‘word’ segments, which are made up of blocks or ‘bits’ of magnetic coating. Each word segment has the same length, with the presence of magnetic material in a bit denoting a ‘1’, and the absence of magnetic material a ‘0’. This thread allows for unique identification of the security elements but is not intended as a publicly verifiable feature.
The need to combine the benefits of a machine readable layer and the public recognition properties of Cleartext® has been recognised and EP-A-0516790 describes a security element comprising a transparent carrier material, a metallic layer with gaps and a magnetic layer disposed above or below the metal layer. The gaps take the form of characters, patterns or other indicia and are located in those areas where no magnetic layer is present, thus ensuring that the negative writing on the thread is readable in transmitted light. The security elements may have an additional magnetic layer, electro-conductive material added to the magnetic layer or magnetic material added to the metallic layer, providing a form of ‘coding’. This security element has the advantage that it combines an easily recognisable feature with a machine-verifiable one.
EP-A-0961996 and EP-A-0938417 disclose further improved security elements comprising a plastic layer, an opaque layer with gaps and a magnetic layer. In common with the security elements described in EP-A-0516790 the gaps are positioned in those areas where no magnetic layer is present so they are visible in transmitted light, but the magnetic layer is additionally periodically spaced with non-magnetic areas to form a coding. The coding may take the form of the codes described in EP-A-0407550, or may be of a different type. Though both EP-A-0516790 and EP-A-0961996 do provide both public and machine-readable properties the current invention seeks to further improve on the machine readable, public recognition and anti-counterfeit properties.